Carpets of crinoids – cousins of the sea-star – stretched their long limbs languidly into the current for morsels of planktonic food. Colorful tropical fish drifted among gracefully spiraling wire corals. Somber-faced grouper hovered warily while jacks and sharks cruised by, curious about the submersibles lights.

Fifty miles south of Mississippi, I was the first human ever to lay eyes on the teeming, thriving, dazzling undersea metropolis that was Salt Dome Mountain. As rich and diverse as Texas Flower Garden Banks National Marine Sanctuary to the west, or Floridas coral reefs to the south, but a little deeper, and totally unexplored.

It was July 29, 2002, and I was a submersible pilot with the Sustainable Seas Expeditions, a joint project of National Geographic and the National Oceanic and Atmospheric Administration, led by my mother, Dr. Sylvia Earle. Fishermen and oilmen have long known the Gulf of Mexico by what they could extract from it with their nets and their drilling rigs. We were there to study it from the inside out.

Salt Dome Mountain is an unexpectedly shallow seamount rising from the depths of the Gulf of Mexico to within 200 feet of the surface. Its just south of the Mississippi coast, just north of where a raging gusher of oil now spews death and destruction with no end in sight. And no beginning in sight either, as the vast majority of this catastrophe is occurring underwater, beyond the reach of television news cameras.

The video below is a compilation of images from my dive eight years ago, posted with permission from Sustainable Seas Expeditions. You can find more videos of the undersea life near the blowout by using Google Ocean.

It remains to be seen when or even whether the raging torrent of oil can be stopped, but even in the best case scenario, the damage already done far exceeds what most of us can yet get our minds around. May it at least not pass unnoticed. And may we at long last consider that the consequences of our actions should be weighed before, and not after, the damage is done.

There, in complete darkness and near-freezing temperatures, the robot’s lights revealed a thriving colony of corals, anemones, fish, crustaceans and other sea life rivaling that of any shallow-water reef in the world. Researchers onboard were elated.

“We flipped on the lights, and there was one of the largest coral reefs in the Gulf of Mexico sitting right in front of us,” said Erik Cordes, a marine biologist at Temple University and chief scientist on the vessel, the Ronald H. Brown.

Nine months later, the warm thrill of discovery has cooled into dread. The reef lies just 20 miles northeast of BP’s blown-out well, making it one of at least three extensive deepwater reefs lying directly beneath the oil slick in the gulf.

Yet it is not the slick that troubles scientists. They fear a more insidious threat: vast plumes of partly dissolved oil apparently spreading in the deep ocean.

The latest research team in the gulf to detect these plumes observed one extending roughly 22 miles northeast of the well site, in the vicinity of at least two major deepwater reefs, including the one discovered last fall. Preliminary images of the plume show layers of it touching the sea floor. Marine scientists have no firm grasp yet on what the impact on the corals will be, but they are bracing for catastrophe.

“The worst-case scenario is that there’s oil coating some of the corals,” Dr. Cordes said. “It would basically suffocate them.”

The composition and distribution of these plumes remain a mystery, and several government research vessels are aggressively pursuing them in the gulf. Scientists believe that the plumes are not pure oil, but most likely a haze of oil droplets, natural gas and the dispersant chemical Corexit, 210,000 gallons of which has been mixed into the jet of oil streaming from the seafloor.

This oily haze could prove highly toxic to coral reefs. Both oil and dispersants, which chemically resemble dishwashing detergent, hamper the ability of corals to colonize and reproduce. And these effects are amplified when the two are mixed.

Studies on the effects of oil and chemicals on coral are limited to the shallow-water variety, however. Essentially no research has been conducted on their slow-growing deepwater cousins. So BP’s spill has prompted scientists to embark on a sudden crash course on the interaction of deep-sea biology with these toxins.

“Everybody’s scrambling,” said Steve W. Ross, a marine biologist at the University of North Carolina, Wilmington, and an expert on deepwater corals. “There’s a lot of evaluation that has to be done.”

But some believe that studies on the impact of oil and dispersants should have been done long ago, given the proliferation of drilling rigs in the Gulf of Mexico.

“Some of these studies were proposed years ago, and the agencies decided not to fund them,” Dr. Ross said. “We’re paying the price for it now.”

The BP Gulf oil spill has now washed ashore in Louisiana, Mississippi, and Alabama, and is expected to reach shore in Florida on Friday: here.

Cut into the continental shelf off the Atlantic coast of the United States is a series of undersea canyons, starting just north of Cape Hatteras, North Carolina and running up past Cape Cod. The canyons plummet down thousands of feet over clay and stone cliffs before reaching the deep ocean bottom. Off the northern end of the canyons’ range, four massive seamounts rise off the ocean floor, part of a chain of extinct drowned volcanoes that stretches down to Bermuda: here.